from operator import itemgetter
from itertools import *
from bisect import *
from collections import *
from heapq import *
from fractions import Fraction
import sys

sys.setrecursionlimit(10 ** 6)

def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def SI(): return sys.stdin.readline()[:-1]
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
int1 = lambda x: int(x) - 1
def MI1(): return map(int1, sys.stdin.readline().split())
def LI1(): return list(map(int1, sys.stdin.readline().split()))
p2D = lambda x: print(*x, sep="\n")
dij = [(1, 0), (0, 1), (-1, 0), (0, -1)]

def isd(d):
    s=set()
    for p in [0,2,4,5,7,9,11]:
        s.add((d+p)%12)
    for t in tt:
        if t%12 not in s:return False
    return True

n=II()
tt=LI()
ans=[]
for d in range(12):
    if isd(d):ans.append(d)
if len(ans)==1:print(ans[0])
else:print(-1)